The present invention is directed, in general, to geophysical exploration and, more specifically, to a system and method for determining an azimuth of a shear wave seismic source.
Most geophysical techniques currently dealing with multi-dimensional seismic data do not discriminate between seismic energies of different orientations, such as the compressional energy or vertical and horizontal shear energies of reflected seismic data systems. In a typical multi-dimensional seismic survey, a multi-mode seismic energy generator may be used to generate a preponderance of one orientation of seismic energy relative to a particular orientation. Then a preponderance of energies orthogonal to the first but relative to the same orientation may also be generated. However, the orientation of the received seismic energy changes at each receiver station due to a difference in orientation between the seismic energy source and each receiver in a multi-dimensional seismic array.
Differently oriented seismic energies may also propagate differently through the subsurface strata based upon the characteristics of the subsurface strata. Anisotropies in the subsurface strata also impact the seismic energies of different orientations, especially shear wave energy. Anisotropic subsurface parameters may be found in the form of thin-bed strata, laminae and bed matrix grains or pores that have a preferential direction caused by deposition or tectonic stress. Another common form of anisotropic subsurface properties are subsurface fractures. Anisotropies cause subsurface parameters such as permeability, shear strength and seismic velocities to have different values in different directions.
Compressional energy waves may generate vertical shear energy waves at subsurface interfaces. Additionally, vertical and horizontal shear waves may acquire significant second-order properties in areas containing subsurface anisotropies that complicate the problem of intermingling but also offer opportunity for analysis if the energies could be segregated. However, the processing of such data is complicated due to the intermingling and therefore not easily discriminated into the differently oriented energies for each source-receiver azimuth. Also, the processing of these components is further complicated since the orientation of the operational modes of the seismic energy source do not generally correspond to the orientation of each receiver in the geophysical data acquisition array.
The mapping of subsurface features may be greatly enhanced by processing the differently oriented seismic energies in a way that accommodates their different attributes. This is especially true in an orientation specific to the azimuths defined by each seismic energy source and receiver pair. Additionally, important rock property information could be ascertained by comparing differences and similarities of the attributes of the appropriately oriented seismic energies.
The orientation of seismic energy from a seismic energy source operating in the field is normally directed in either an inline or a crossline direction. This is due to field operating complexities and seismic energy source constraints. This situation often results in a less-than-desired level of seismic energy occurring in a particular direction than is really needed to clearly illuminate a subsurface event. This situation may not be fully appreciated until post-field processing has occurred sometimes requiring the collection of more field data to rectify. Additionally, orienting the seismic energy source in a normal field survey environment to provide other than inline or crossline seismic energy is typically difficult, at best.
Accordingly, what is needed in the art is a way to more effectively orient and segregate seismic source energy in seismic surveying situations.
To address the above-discussed deficiencies of the prior art, the present invention provides a directional assembly for determining an azimuth of a seismic energy source. In one embodiment, the directional assembly includes a mount configured to be coupled to the seismic energy source, a rotatable mass assembly coupled to the mount, a compass rose coupled to one of the mount or the rotatable mass assembly and a direction reference coupled to the other of the mount or the rotatable mass assembly. The compass rose is registered with the direction reference to provide a direction orientation of the rotatable mass assembly with respect to the mount.
In a particular embodiment, the compass rose is coupled to the mount and the direction reference is coupled to the rotatable mass assembly. In an alternative embodiment, the compass rose is coupled to the rotatable mass assembly and the direction reference is coupled to the mount. In either of these embodiments, the direction reference is magnetic north. Alternatively, the direction reference may correspond with a cross line direction, an inline direction or to another advantageously selected direction.
In yet another embodiment, the compass rose includes a signal transmitter and the direction reference includes a signal receiver. Alternatively, the compass rose may include a signal receiver and the direction reference may include a signal transmitter. The signal transmitter is located adjacent an outer circumference of the compass rose and corresponds to a degree of rotation about the circumference. A direction indicator is associated with the direction reference and is configured to provide data regarding the orientation of the rotatable mass assembly. Further, a communication network, coupled to the direction indicator, is configured to transmit the orientation data to a remote recording location.
The present invention also provides a seismic exploration system. In an advantageous embodiment, the system includes a seismic energy source employing a support structure, a directional assembly coupled to the support structure that includes a mount coupled to the support structure, a rotatable mass assembly coupable to the mount, a compass rose coupled to one of the mount or the rotatable mass assembly, a direction reference coupled to another of the one of the mount or the rotatable mass assembly, receivers located on a terrain and a seismic recorder system. The compass rose is registered with the direction reference to provide a direction orientation of the rotatable mass assembly with respect to the mount.
In another aspect, the present invention provides a method of orienting a seismic source. In one exemplary embodiment, the method includes registering a compass rose with a direction reference to orient a rotatable mass assembly of a seismic source with respect to a mount of the seismic source, wherein the compass rose is coupled to either the mount or the rotatable mass assembly with the direction reference being coupled to the other of the mount or the rotatable mass assembly.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiments as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.